In many computing systems, a processor or controller in one chip is coupled through a bus to memory, which may be on a separate chip or more often is part of a separate module. A memory module typically includes several memory chips and a memory controller chip all on a small printed circuit board that plugs into the board that carries the controller or the processor. A separate memory module allows the memory to be made by a company other than the one that made the processor or controller and it allows the amount of memory to be changed for different purposes. In many computer applications, the processor is not coupled directly to the memory module, instead the processor is coupled to a memory controller hub through one bus and then the memory controller hub is coupled to one or more memory modules through a second bus. As memories grow larger and processors grow faster, the buses that connect the memory to the processor must increase in bandwidth to be able to carry ever more data in less time. Similar demands are placed on other buses within electronic systems.
In computer applications, to save on the number of pins and traces required by these buses to support the bandwidth of ever faster microprocessor cores, the data rates on the busses that connect the cores to memory, graphics and peripherals scale to higher rates. Rates above 5 Gbps (Gigabits per second) per bus lane are projected for the near future. Rates of 5 Gbps and above are not easily supported using practical cost-competitive circuit boards that use low cost connectors, sockets and materials (e.g. FR4). Such materials introduce distortions and interference in high frequency signals that make it difficult for the chip on the other side of the bus to read the signal that it receives. In order to compensate for these low cost materials, equalization has been used on both parallel and serial buses.
The equalization may be performed using an equalizer on the transmitter side of the bus. Such an equalizer anticipates the changes that will occur in the transmitted signal as it travels across the bus and modifies the signal before it is transmitted in order to compensate. An example of such a compensation is de-emphasis. In other words, the transmit side equalizer pre-distorts the transmitted waveform to account for the distortion that will occur along the entire channel. This can minimize intersymbol interference, loss and other effects at the receiver at the far end of the bus. Alternatively, an equalizer may be placed on the receiver side. A receiver side equalizer will compensate for changes that occur to the signal after it propagates across the bus before a receiver tries to interpret the signal. For more extreme situations or higher accuracy, a combination of transmitter and receiver equalization may be used.
To test and debug microprocessors, memories and other chips and the busses that connect them, logic probes are used. The probes connect to validation systems and tools. Traditionally, logic probes are a simple metal pin with a handle that can detect the voltage or current in a copper line on a printed circuit board or on a pin of a chip. Boards and chips, as they have become more complex, have also been modified to provide special pads that logic probes can use to make electrical contact. The cost of the pads and the cost of the logic probes can, however, have an impact on the cost of the boards and the chips. In addition, with higher data rates, lower voltages and lower currents, the probes have an increasingly large effect on the integrity of the signals in the busses and on the pins. If the signal is perturbed by the probe, then the validation systems and tools will not generate accurate results. At worst, the components will not operate properly rendering many tests useless.
In order to minimize the impact of the probe on the operation of a chip or a bus, a probe may be attached close to the transmitter at a microprocessor, chipset hub, or memory chip pin. In other cases, the logic probes may be attached to connectors along the bus traces, at the backside of a socket or the backside of through-hole vias in a connector. In order to enhance the signal strength detected by the probe, logic probes may be placed much closer to a transmitter than to a receiver at the far end of the bus. For high bandwidth busses, when the probe is at the receive end of the bus, the received signal is very weak, making it hard for the probe to pick up the signal and increasing the negative impact on the receiver. When the probe is positioned at points closer to the transmitter, the logic probe may pick up a signal that has been pre-distorted by a transmitter equalizer. However, the signal may include transmit equalizer artifacts caused by over equalization because the transmitter equalization (TX EQ) was designed to compensate for the loss at the end of the channel. It therefore overcompensates for probes positioned closer to the transmitter. The logic probe receivers must somehow absorb the link margin degradation associated with the transmit side equalization artifacts. This increases the complexity of the validation systems and reduces the accuracy of test results.